1. Field of the Invention
The present invention relates to a catalyst deterioration detecting apparatus for an internal combustion engine and, more particularly, to a catalyst deterioration detecting apparatus for an internal combustion engine provided in a hybrid vehicle employing both an internal combustion engine and an electric motor as power sources.
2. Description of the Related Art
A three-way catalyst of a catalytic converter disposed in an exhaust passage of an internal combustion engine has an oxygen storage function taking up oxygen when the air-fuel ratio is on the fuel-lean side of a stoichiometric air-fuel ratio and, when the air-fuel ratio turns to a fuel-rich side of the stoichiometric air-fuel ratio, causing oxygen previously taken up during the fuel-lean state to react with hydrocarbons and carbon monoxide present in the exhaust gas. If the oxygen storage function of the catalyst deteriorates, the emission control performance becomes degraded. That is, hydrocarbons and carbon monoxide are let out to the downstream side of the catalyst, so that maintenance of good emission quality becomes difficult. In order to maintain good emission quality, it is necessary to detect a degree of deterioration of the catalyst and determine whether the catalyst has deteriorated excessively based on the degree of deterioration and, when it is determined that the catalyst has deteriorated excessively, to visually notify a driving person of the deterioration of the catalyst and change the air-fuel ratio control in accordance with the degree of deterioration of the catalyst.
Various apparatuses for detecting the degree of deterioration of a three-way catalyst based on the degree of deterioration of the oxygen storage function of the catalyst have been proposed. For example, a catalyst deterioration degree detecting apparatus disclosed in Japanese Patent Application Laid-Open No. HEI 6-159048 detects a degree of deterioration of a catalytic converter in the following manner. After switching the air-fuel ratio to the rich side following execution of a fuel cut control of the engine, the catalyst deterioration degree detecting apparatus calculates an amount of oxygen released from the catalytic converter during a period of time up to a time at which the air-fuel ratio downstream of the catalyst actually switches from the lean side to the rich side, based on a total amount of exhaust gas that passes through the catalytic converter during that period and the rich side-switched air-fuel ratio. Considering the calculated released amount of oxygen as indicative of an oxygen adsorbing capacity of the catalytic converter, the apparatus detects a degree of deterioration of the catalytic converter.
The aforementioned apparatus can be applied to a hybrid vehicle that selectively uses power from an internal combustion engine and power from an electric motor and temporarily stops the engine when using power from the motor. However, in such case, it is difficult to determinate a deterioration of a catalyst with high precision. The reason is that if the engine fuel cut is continued for a long time, the temperature of the catalyst is decreased so that the catalyst cannot store oxygen up to the maximum oxygen adsorbing capacity thereof even in the presence of an excess amount of oxygen. Conversely, if the fuel cut duration is short, there is not provided a time needed for sufficient adsorption of oxygen to the catalyst. In such cases, an excessive degree of deterioration of the catalyst is detected although the amount of oxygen stored in the catalyst is less than the maximum oxygen adsorbing capacity of the catalyst, so that the determination regarding deterioration of the catalyst may not be performed with high precision.